The long term goal of these studies is an increased understanding of the nature of cortical auditory processing for speech. In this project, neurophysiological correlates of speech stimuli will be analyzed to determine whether cortical electrical activity reflects the perceptual phenomenon of categorical perception. Specifically, cortical evoked potential responses (EPs) to speech sound signals which vary only in voice-onset-time (VOT) will be obtained in adult patients with known seizure foci. Evoked potentials will be obtained directly from the cortical surface via surgically implanted strip electrodes placed for clinical purposes. This method of data collection is a particularly powerful one in that EP amplitude, localization and detailed morphology have been shown to be better defined than when subject to attenuation by the skull and scalp. Preliminary studies obtained in the first year of the project have yielded intriguing results which suggest marked asymmetry in cortical response to speech stimuli and intrahemispheric spatial localizations of EP responses to speech stimuli. Also striking was the strong suggestion of EP latency differences to speech stimuli which differ along the acoustic dimension of VOT. In the current proposal, the issues of EP asymmetry and spatial localization will continue to be addressed. The study of morphological differences in EPs to speech stimuli which differ in VOT will be expanded to study whether EP differences reflect phonological, phonetic or purely acoustic distinction between speech stimuli. Also included will be an analysis of EP responses to non-speech stimuli which differ along a similar acoustic feature (tone onset time). The electrophysiological indices of categorical acoustic processing will be compared to behavioral capabilities in categorical perception paradigms, to location of epileptic foci and to side of language lateralization. These studies are designed to shed light on the cortical processing of speech and non-speech stimuli. Approaches to the study of neurobiological bases of speech perception are limited and the opportunity to study direct cortical EP activity to speech presents a special situation with which to expand our knowledge about cortical responses to complex acoustic stimuli and the nature of speech sounding encoding.